Actuator and valve assembly

ABSTRACT

An actuator for selectively moving a part relative to another part includes a shape memory wire, which contracts when heated. One portion of the shape memory wire is fixed to a first body, and another portion of the shape memory wire is coupled to a second body, which is movable relative to the first body. When heated the shape memory wire contracts and moves the second body in a direction of travel from a first position relative to the first body toward a second position. The actuator further includes a magnetic field for generating a magnetic force on the second body. The magnetic force acts on and moves the second body in the direction of travel to the second position after the shape memory wire initially contracts, and further decouples the second body from the shape memory wire when applying the magnetic force to the second body to thereby unload the load on the shape memory wire when the second body is moved to the second position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. copendingapplication Ser. No. 11/396,671, filed Apr. 3, 2006, entitled ACTUATORAND VALVE ASSEMBLY by Applicants Mark William Friedman and Robert BrianKnepple, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to an actuator and, more particularly, toan actuator that incorporates the use of shape memory wires and that isparticularly suitable for use in a valve.

Shape memory wires or “muscle wires” contract when heated; however, whenshape memory wires are subjected to constant or increasing loads duringtheir contraction their strokes will be limited or their size will needto be increased in order to prevent the stresses in the wires fromexceeding the wire failure stress. As a result, their use in certainapplications has been limited.

Accordingly there is a need for an actuator that can take advantage ofthe properties of shape memory wires and yet avoid their inherentlimitations.

SUMMARY

The present invention is directed to an improved actuator that can beused in a valve which can potentially reduce the size, weight and,ultimately, the cost of the valve. As will be more fully describedbelow, the actuator incorporates the use of shape memory wires to takeadvantage of their contraction properties but at the same time avoidstheir short comings.

In one form of the invention, a valve includes a valve body, with apassageway forming a first port and a second port, and a valve stemlocated in the passageway and supported for reciprocal movement in thepassageway between a first position and a second position. Thepassageway is open to permit fluid communication between the ports whenthe valve stem is moved to one of the two positions thereby opening thevalve and is closed between the two ports when the valve stem is movedto the other of the two positions. The valve also includes an actuator,which selectively moves the valve stem in a direction of travel betweenthe two positions for opening or closing the valve. The actuatorincludes a shape memory wire, which contracts when heated. One end ofthe shape memory wire is fixed relative to the valve body and anotherend of the shape memory wire is coupled to the valve stem in thedirection of the travel of the valve stem so that when heated thecontraction of the shape memory wire moves the valve stem in thedirection of travel toward the second position. The actuator alsoincludes a force generator, such as a magnetic field, which generates amagnetic force. The magnetic force acts on the valve stem and moves thevalve stem in the direction of travel, and optionally independently ofthe shape memory wire, to the second position and seats the valve stemat the second position after the shape memory wire initially contractsto thereby open or close the valve.

In one aspect, the magnetic force reduces the load on the wire when thevalve stem is moved to the second position. For example, the valve stemmay be movable with respect to the memory wire in the direction oftravel, wherein the magnetic force decouples the valve stem from theshape memory wire to thereby unload the wire.

In another aspect, the valve includes a first magnet mounted to the bodyand a second magnet mounted to the valve stem. The magnets generate themagnetic force. For example, the first magnet may comprise an annularmagnet or a plurality of magnets mounted to the body about the valvestem. Similarly, the second magnet may comprise an annular magnet or aplurality of magnets mounted to the valve stem.

In another aspect, the passageway further defines a third port, whichthird port is in fluid communication with the second port when the valvestem is in one of the positions and is sealed from other two ports whenthe valve stem is moved to the other position to thereby form a 3-wayvalve.

In yet a further aspect, the valve includes a pair of the shape memorywires. For example, one of the shape memory wires may be configured topull the valve stem in one direction of travel, while the other shapememory wire may be configured to pull the valve stem in the oppositedirection of travel.

In yet another aspect, the shape memory wire is aligned in the directionof travel. Alternately, the shape memory wire may be angled with respectto the direction of travel wherein only a component of the contractionis aligned with the direction of travel.

According to another form of the invention, a valve includes a valvebody, with a passageway that forms a first port and a second port, and avalve stem that is located in the passageway and supported forreciprocal movement in the passageway between a first position and asecond position. The passageway is open to permit fluid communicationbetween the two ports when the valve stem is moved to the first positionthereby opening the valve. The passageway is closed between the twoports when the valve stem is moved to the second position whereby thevalve assembly is closed. The valve also includes an actuator thatselectively moves the valve stem in two directions of travel between thefirst and second positions and which includes a first shape memory wireand a second shape memory wire. One end of the first shape memory wireis fixed at a first location relative to the body and another end of thefirst shape memory wire is coupled to the valve stem in a firstdirection of travel of the valve stem. When heated the first shapememory wire contracts and moves the valve stem toward the first locationin the first direction of travel. One end of the second shape memorywire is fixed at a second location of the body, and another end of thesecond shape memory wire is coupled to the valve stem in a seconddirection of travel. When heat is applied to the second shape memorywire, contraction of the second shape memory wire moves the valve stemtoward the second location of the body in the second direction of travelopposed from the first direction of travel. The actuator furtherincludes a magnetic field that generates a magnetic force, which acts onand moves the valve stem to the first position and seats the valve stemat the first position after the first shape memory wire initiallycontracts. The magnetic force also moves the valve stem to the secondposition and seats the valve stem at the second position after thesecond shape memory wire initially contracts.

In one aspect, the magnetic force reduces the load on the wires when themagnetic force moves the valve stem to the first or second position. Forexample, the valve stem may be movable relative to the first shapememory wire in the first direction of travel wherein the valve stem canmove in the first direction of travel independently relative to thefirst shape memory wire. Similarly, the valve stem may movable relativeto the second shape memory wire in the second direction of travelwherein the valve stem can move independently relative to the secondshape memory wire. In this manner, when the magnetic force is applied tothe valve stem, the magnetic force decouples the valve stem from therespective shape memory wire.

In a further aspect, the magnetic force is generated by a first magnetmounted to the body and a second magnet mounted to the valve stem.

According to another aspect, the first magnet is mounted to the bodyabout the valve stem. The second magnet is mounted to the valve stemradially inward of the first magnet and is offset relative to the firstmagnet in the first direction of travel when the valve stem is moved tothe first position and is offset relative to the first magnet in thesecond direction of travel when the valve stem is moved to the secondposition. In this manner, when the valve stem is in the first position,the two magnets generate a repulsion force that latches the valve stemat the first position. Similarly, when the valve stem is in the secondposition, the two magnets generate a repulsion force that latches thevalve stem in the second position. Thus, when the first shape memorywire contracts, it must initially overcome the latching force betweenthe two magnets to move the valve stem in the first direction of travelaway from the second position toward the first position. When the secondmagnet moves past the first magnet in the first direction of travel itthen generates the repulsion force therebetween in the first directionof travel to move the valve stem to the first position and seat thevalve stem at the first position. When the second shape memory wirecontracts it must initially overcome the latching force between the twomagnets to move the valve stem away from the first position toward thesecond position in the second direction of travel. When the secondmagnet moves past the first magnet in the second direction of travel itthen generates the repulsion force therebetween in the second directionof travel to move the valve stem to the second position and seat thevalve stem at the second position.

In a further aspect, the first magnet comprises an annular magnet, andthe second magnet comprises an annular magnet.

In yet another aspect, the first shape memory wire comprises a firstpair of shape memory wires, and the second shape memory wire comprises asecond pair of shape memory wires. For example, the first pair of shapememory wires may be arranged in a first plane, with the second pair ofshape memory wires being arranged in a second plane, wherein the firstplane is angled with respect to the second plane. In the illustratedembodiment, the first plane is generally orthogonal to the second plane.

In another aspect, the valve stem includes an elastomeric body forseating against the first position or the second position in thepassageway.

According to yet another aspect, the shape memory wires extend throughthe valve stem. For example, the valve stem may include a transversepassage for each wire.

In yet another form of the invention, an actuator for selectively movinga first body in a direction of travel relative to a second body includesa shape memory wire, which contracts when heated, and a force generator,such as a magnetic field. One end of the shape memory wire is fixed tothe first body. The other end of the shape memory wire is coupled to thesecond body in the direction of travel, which second body is movablerelative to the first body. When heated, the shape memory wire contractsand pulls the second body in the direction of travel from a firstposition relative to the first body toward a second position relative tothe first body. The force generator generates a force on the secondbody, which acts on and moves the second body in the direction of travelto the second position after the shape memory wire initially contractsindependently of the shape memory wire to thereby reduce the load on theshape memory wire when the second body is moved to the second position.

In one aspect, the actuator further includes a first magnet mounted tothe first body and a second magnet mounted to the second body, with themagnets generating the force. For example, the first magnet may comprisean annular magnet or a plurality of magnets mounted to the first bodyabout the second body. Similarly, the second magnet may comprise anannular magnet or a plurality of magnets mounted to the second body.

In a further aspect, the first magnet is mounted to the first body aboutthe second body, and the second magnet is mounted to the second bodyradially inward of the first magnet. The second magnet is offset in anopposed direction of travel relative to the first magnet when the secondbody is moved to the first position so as to generate a repulsion forcethat latches the second body in the first position. When the shapememory wire contracts and moves the second body away from the firstposition toward the second position, the second magnet moves past thefirst magnet in the first direction of travel and generates a magneticrepulsion force therebetween in the first direction of travel to movethe second body to the second position.

In another aspect, the shape memory wire comprises a first shape memorywire and the direction of travel comprises a first direction of travel.The actuator further includes a second shape memory wire, with one endof the second shape memory wire fixed to the first body and another endof the second shape memory wire coupled to the second body in a seconddirection of travel opposite the first direction of travel such thatwhen the second shape memory wire is heated and contracts the secondshape memory wire moves the second body in the second direction oftravel.

According to yet another aspect, the first shape memory wire comprises afirst pair of shape memory wires arranged in a first plane, and thesecond shape memory wire comprises a second pair of shape memory wiresarranged in a second plane, which is angled with respect to the firstplane.

In yet a further aspect, the shape memory wires extend through thesecond body.

In yet another form of the invention, a method of moving a first partrelative to a second part includes providing a shape memory wire, whichcontracts when heated, fixing one end of the shape memory wire to afirst part, and coupling the other end of the shape memory wire to asecond part, which is movable relative to the first part. The wire isheated to contract the wire to thereby apply a first pulling force onthe second part and move the second part relative to the first part froma first position toward a second position in a direction of travel. Asecond force is generated and applies a second pulling force on thesecond part to move the part to the second position in the direction ofand independently of the shape memory wire wherein the second forcereduces the load on the shape memory wire to thereby reduce the stresson the wire.

In one aspect, the second force is generated by a magnetic field thatproduces a magnetic force. The second part is then pulled with themagnetic force. For example, the magnetic force may be generated bymounting a first magnet to the first part and mounting a second magnetto the second part and orienting their respective poles to generate arepulsion force between the magnets.

In yet a further aspect, a second shape memory wire is provided. One endof the second shape memory wire is fixed to a second portion of thefirst part, and the other end of the shape memory wire is coupled to thesecond part in a second direction of travel. When heated, the secondshape memory wire moves the second part relative to the first part inthe second direction of travel away from the second position to thefirst position. The second force is then applied to the second part tomove the second part to the first position independently of the secondshape memory wire wherein the second force reduces the load on thesecond shape memory wire to thereby reduce the stress on the secondshape memory wire.

Accordingly, the present invention provides a valve and actuator whichincorporate the use of shape memory wires in a manner to reduce thestresses in the shape memory wires while being able to take betteradvantage of their contraction properties than heretofore known.

These and other objects, advantages, purposes, and features of theinvention will become more apparent from the study of the followingdescription taken in conjunction with the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of the valve and actuator of thepresent invention;

FIG. 2 is an enlarged cross-section view of the valve and actuator ofFIG. 1;

FIG. 3 is a similar cross-section view to FIG. 2 taken along aperpendicular plane to FIG. 2;

FIG. 4 is a similar view to FIG. 2 illustrating the valve in a closedstate;

FIG. 5 is a similar cross-section to FIG. 2 illustrating the valve in anopen state; and

FIG. 6 is a force versus stroke diagram for the force generator of theactuator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the numeral 10 generally designates a valve of thepresent invention. As will be more fully described below, valve 10 iselectrically actuated by an improved actuator that incorporates the useof one or more shape memory wires to allow for a more compact valvearrangement and, further, reduces the weight of the valve, while at thesame time reducing the stress in the shape memory wires. In addition,the actuator may eliminate the need for a solenoid, which reduces theweight and size as noted, and also the cost of the valve. While theseadvantages may be achieved with the present invention, these advantagesshould not be construed as limiting in any way.

As best seen in FIGS. 2-3, valve 10 comprises a 3-way valve though itshould be understood from the following description that the actuatormay be used on a 2-way or 4-way valve or on other devices that requireactuation. For example, the actuator may be used as a replacement for asolenoid or any thing that acts like a solenoid, such as a cylinder,including pneumatic or hydraulic cylinders.

Referring to FIGS. 1-5, valve 10 includes a valve body 12 with apassageway 14 that forms a first port 16, a second port 18, and a thirdport 20. The arrangement and locations of the ports may be varied but inthe illustrated embodiment are arranged so that ports 16 and 20 arelocated on one side of valve body 12, while port 18 is located onanother side of valve body 12. Located in passageway 14 is a valve stem22. Valve stem 22 is supported for reciprocal movement in passageway 14and includes a poppet 24, which is formed from an elastomeric materialfor sealing and closing the fluid communication between the ports tothereby open or close the valve. Stem 22 is sealed in passageway 14 by apair of seals, such as U-cup seals 23 a and 23 b. As would beunderstood, poppet 24 may be formed from a monolithic elastomeric bodyor from several separate elastomeric bodies depending on theconfiguration of the valve.

In the illustrated embodiment, poppet 24 is formed from a single,unitary elastomeric body that is dimensioned such that when valve stem22 is moved to a first position (shown in FIG. 4) within passageway 14poppet 24 seals against seat 28 to seal port 16 to block fluidcommunication between port 16 and port 18 to thereby close valve 10.When valve stem 22 is moved to its second position (shown in FIG. 5),poppet 24 seals against seat 30 to seal and close communication betweenport 20 and port 18, but allow fluid communication between ports 16 and18 to thereby open the valve.

To move valve stem 22 between its two positions, valve 10 includes anactuator 32. Actuator 32 includes two or more “memory” wires 34 and 36and optionally two sets or pairs of shape memory wires 34, 35 and 36,37. However, it should be understood that the number of wires and numberof sets or pairs of wires may be increased or decreased. Shape memorywires are formed from a nickel titanium alloy and designed to contractwhen heated to a specific transition temperature and should not beextended until the temperature falls below the transition temperature.Shape memory wires are typically heated to their transition temperatureby directing an electrical current through the wires and then allowed tocool below their transition temperatures by stopping the current flowthrough the wires. While the use of shape memory wires is well known,their application has been limited by the excessive stress levels thatcan be created in the wires if the wires are subject to excessive stressthroughout their contraction, especially at the end of theircontraction. As will be more fully described below, in the presentinvention the actuator maintains the stress levels in the wires at lowerlevels by unloading the wires during their contraction and at the end oftheir contraction, which, therefore, decreases the load on the wirethroughout their stroke.

As best seen in FIGS. 2 and 3, shape memory wires 34, 35 and 36, 37 arefixed at their respective ends 34 a, 35 a and 36 a, 37 a relative tovalve body 12, while their opposed ends 34 b, 35 b and 36 b, 37 b arecoupled to valve stem 22 at least in one direction—generally in thedirection of contraction. Electrical current is selectively applied tothe wires to thereby heat the wires when actuation of the valve isdesired, as will be more fully described below. For example, the ends ofthe wires may be coupled to electrical conductors, which then are routedout from the valve body for coupling to a power supply that iscontrolled for example by a suitable controller.

In the illustrated embodiment, ends 34 a, 36 a are fixed relative tobody 12 by a wire anchor 38, which is threaded into one end of body 12,and a retainer 40: Retainer 40 is secured to anchor 38 by a retainerclip 40 a, such as an E-shaped clip. Ends 35 a and 37 a are fixed tobody 12 at an opposed end or location of body 12 by a second wire anchor42, which is similarly threaded into the opposed end of body 12, and aretainer 44. The opposed ends 34 b, 36 b of wires 34 and 36 are thencoupled to stem 22 in at least one direction—for example in theirrespective direction of contraction by a wire retainer 46, which issecured to end 22 a of stem 22 by a retainer clip 46 b, such as anE-shaped clip retainer. In the illustrated embodiment, wires 34 and 36and 35 and 37 are generally aligned with the axis of travel of the valvestem so that there is a direct correlation between the magnitude of thewires' contraction and the distance moved by the valve stem. However, itshould be appreciated that the shape memory wires may be angled to theaxis of travel so that only a component of the wires' contraction willinduce travel in the valve stem.

In one form, stem 22 can be decoupled from and movable relative to wires34 and 36 in the direction of travel of valve stem 22 toward anchor 38.The opposed ends 35 b, 37 b of wires 35 and 37 are similarly coupled tostem 22 in the opposite direction of travel by another wire retainer 47,which is secured to the other end of stem 22 by a retainer clip 47 a,such as an E-shaped clip retainer. In addition, valve stem 22 canoptionally be decoupled from and movable with respect to wires 35 and 37in the direction of travel toward anchor 42.

In this manner, one end of wires 34 and 36 is fixed relative to body 12,while the other ends of wire 34 and 36 are coupled to valve stem 22 sothat when wires 34 and 36 contract, stem 22 moves toward wire retainer40 in a first direction of travel. Similarly, one end of wires 35 and 37is fixed relative to body 12, while the other ends of wires 35 and 37are coupled relative to valve stem 22 so that when wires 35 and 37contract; stem 22 moves toward wire retainer 44 in the second directionof travel opposite the first direction of travel. Further, as noted,valve stem 22 is optionally movable with respect to wires 34 and 36 inthe first direction of travel toward retainer 40 and is movable withrespect to wires 35 and 37 in the opposite direction of travel. As wouldbe appreciated, when wires 34 and 36 are heated, wires 34 and 36 movestem 22 from a position shown in FIG. 3 (where poppet 24 seats againstseat 30 and where poppet 24 closes fluid communication between ports 20and 18 but allows fluid communication between ports 16 and 18 to therebyopen the valves) toward another position shown in FIG. 4 (where poppet24 seats against seat 28 to thereby close fluid communication betweenport 16 and port 18 to thereby close the valve). In contrast, when wires35 and 37 contract, stem 22 moves away from seat 28 toward seat 30 tothereby close fluid communication between port 20 and port 18 but openfluid communication between port 16 and 18 to thereby open the valve.However, as will be described below, wires 34, 35 and 36, 37 do not seatthe valve stem poppet at the seats. This is done instead by a forcegenerator 50.

To reduce the force on the respective wires during each respectivewire's contraction and, further, at the end or latter half of theirrespective contractions, actuator 32 further includes force generator50. Force generator 50 generates latching forces to latch valve stem 22at its respective positions where poppet 24 is seated at either seat 28or seat 30. These latching forces are greatest when the poppet is seatedat one of the seats. However, once a pulling force (i.e. when the shapewires contract) is applied to the valve stem in a direction opposed fromthe latching force and the valve stem moves away from its seatedposition against one of the seats, the latching force diminishes so thatthe load on the respective wires reduces. Referring to FIG. 6, thelatching force is greatest at the beginning of the stroke and thendecreases to zero and thereafter becomes a driving force to urge thevalve stem in the direction of the respective wires' contraction.Further, as previously noted, valve stem 22 can optionally be decoupledin the direction of travel from the contracting wires so that the forcegenerator 50 decouples the valve stem from the contracting wire andtakes over and moves the valve stem to its other seated positionindependent of the contracting wire and further latches the valve stemin its other seated position. Even when valve stem 22 is not decoupledfrom the wire, the load on the contracting wire will be reduced once thelatching force transitions into the driving force on the valve stem. Thepoint at which the force generator takes over can vary, but in theillustrated embodiment the force generator 50 is configured to move thevalve stem over its latter half of the stroke to thereby reduce the loadon the respective wires at the end or latter half of their respectivecontractions.

Referring again to FIGS. 1-5, in the illustrated embodiment, forcegenerator 50 generates a magnetic force that acts on the valve stem 22after the respective wires initially contract to move the valve stemfrom its seated position at the respective seats 28 and 30.Consequently, wires 34, 35, and 36, 37 are loaded only over a portion ofthe valve stem's stroke. For wires 34 and 36 it is that portion of thestroke when the valve stem initially moves away from seat 30 and towardseat 28. Wires 35 and 37 are loaded over that portion of the valve stemstroke when the valve stem moves away from seat 28 toward seat 30. Themagnetic force then moves the valve stem over the remaining portion ofthe valve stem stroke when the poppet is moved to a respective seat andthen seated at the respective seat, where the magnetic forces are attheir highest levels.

The magnetic force is generated by a pair of magnets 52 and 54. Magnet52 is mounted in a fixed position relative to valve body 12. Magnet 54is mounted in a fixed position relative to valve stem 22 such thatmagnet 54 is movable relative to magnet 52. In the illustratedembodiment, magnet 52 forms an outer magnet, while magnet 54 forms aninner magnet. Magnets 52 and 54 are oriented with their magnetic polesin the same direction—for example at the top 56, 58 and bottom sides 60,62 of the magnets. Thus, the magnets generate a magnetic repulsion forcebetween the two magnets that pushes the magnets away from each other andfurther in an upward or downward direction when they are verticallyoffset from each other (as viewed in FIGS. 2-5).

Referring to FIGS. 2 and 3, when poppet 24 is seated against seat 28magnets 52 and 54 are arranged such that magnet 54 is above magnet 52.When poppet 24 is seated against seat 30 magnets 52 and 54 are arrangedsuch that magnet 54 is below magnet 52. Therefore, when stem 22 isinitially pulled away from its first position where poppet 24 is seatedagainst seat 28 magnet 54 initially moves downward (as viewed in FIG. 4)relative to magnet 52 away from upper end 12 a of body 12 (as viewed inFIGS. 2-5) in the downward direction of travel. The magnetic forcebetween the magnets will initially resist this downward movement (seeFIG. 4) of magnet 54 and, hence, of valve stem 22. This resistance,however, will decrease until magnets 52 and 54 are aligned, in whichcase the resistance becomes insignificant and over a short distancereduces to zero. As magnet 54 moves past or below the aligned position,the magnetic force will then generate a downward force urging magnet 54in the downward direction of travel, which downward force moves valvestem 22 to its second position where poppet 24 seats against seat 30(FIG. 5). In this valve stem configuration, magnets 52 and 54 work in asimilar manner but in a reversed direction.

In the configuration shown in FIG. 5, magnets 52 and 54 initially resistupward movement of valve stem 22 in the upward direction of travel, butonce magnet 54 moves above the aligned position, magnets 52 and 54 liftvalve stem 22 so that poppet 24 seats against seat 28. It should beunderstood that the references to “above”, “below”, “upward” or“downward” are used are used in reference to the orientations shown inthe illustrated drawings for nomenclature purposes only and, hence, arenot limiting.

Wires 34 and 36 are initially contracted to overcome the downwardresistive force of magnets 52 and 54 when valve stem 22 is in itslowermost position as viewed in FIG. 5—once magnet 54 is moved above thealigned position of the magnets, the magnetic force will essentially“take over” and move the valve stem to its uppermost position wherepoppet 24 is seated against seat 28.

Similarly, wires 35 and 37 are contracted to apply a downward force onvalve stem 22 against the magnetic resistive force generated by themagnets when stem 22 is in its uppermost position with poppet 24 seatedagainst seat 28. Once magnet 54 is moved below the aligned position ofthe magnets, the downward magnet force generated by the magnetseffectively “takes over” and moves the valve stem 22 downward so thatpoppet 24 seats against seat 30. Thus, the magnets form a magnetic latchand, further, unload the wires during their contraction and, moreover,at the end of their contractions.

Referring to FIG. 6, the magnetic force generated by magnets versus thestroke of the valve stem varies depending on the strength and size ofthe magnets. In the illustrated embodiment, magnets 52 and 54 eachcomprise a pair of magnets 52 a, 52 b and 54 a, 54 b. It should beunderstood, therefore, the number of magnets may be increased ordecreased depending on the desired stroke and force needed at the end ofthe stroke.

Consequently, the actuator of the present invention is able to takeadvantage of the contraction properties of shape memory wires whilereducing the load or unloading the load on the shape memory wires duringthe stroke and at the end of their contraction where the wires aresusceptible to failure.

While several forms of the invention have been described, other formswill now be apparent to those skilled in the art. Therefore, it will beunderstood that the embodiments shown in the drawings and describedabove are merely for illustrative purposes, and are not intended tolimit the scope of the invention, which is defined by the claims, whichfollow as interpreted under the principles of patent law including thedoctrine of equivalents.

We claim:
 1. An actuator for selectively moving a part relative toanother part; said actuator comprising: a shape memory wire, said wirecontracting when heated, one portion of said shape memory wire fixed toa first body, another portion of said shape memory wire coupled to asecond body, said second body movable relative to said first body, whenheated said shape memory wire contracts and moves said second body in adirection of travel from a first position relative to said first bodytoward a second position; a magnetic field, said magnetic fieldgenerating a magnetic force on said second body, said magnetic forceacting on and moving said second body in said direction of travel tosaid second position after said shape memory wire initially contracts;and said magnetic force decoupling said second body from the forceapplied by said shape memory wire and moving said second body relativeto said shape memory wire when applying said magnetic force to saidsecond body to thereby unload the load on the shape memory wire when thesecond body is moved to said second position.
 2. The actuator accordingto claim 1, wherein said memory wire is coupled to said second body insaid direction of travel, and said second body is decoupled from saidmemory in said direction of travel wherein when said magnetic force isacting on said second body, said second body moves in said direction oftravel and relative to and along said memory wire to thereby unload saidshape memory wire.
 3. The actuator according to claim 1, furthercomprising a first magnet mounted to said first body and a second magnetmounted to said second body, and said magnets generating said magneticforce.
 4. The actuator according to claim 3, wherein said magnetic forceacting on said second body to move said second body comprises a magneticrepulsion force.
 5. The actuator according to claim 1, wherein saidactuator comprises a pair of said shape memory wires, said shape memorywires being parallel to each other and parallel to said direction oftravel wherein the longitudinal contraction of said wires issubstantially equal to the longitudinal movement of the second body insaid direction of travel when said wires are acting on said second body.6. The actuator according to claim 1, wherein said shape memory wirecomprises a first shape memory wire and said direction of travelcomprises a first direction of travel, said actuator further comprisinga second shape memory wire, said second shape memory wire contractingwhen heated, one portion of said second shape memory wire fixed at asecond location of said first body and another portion of said secondshape memory wire coupled to said second body wherein when heated saidsecond shape memory wire contracts and moves said second body towardsaid second location of said body in a second direction of travelopposed from said first direction of travel.
 7. The actuator accordingto claim 1, further in combination with a valve, said valve having avalve stem, and said actuator selectively moving said valve stem.
 8. Anactuator for selectively moving a part relative to another part, saidactuator comprising: a shape memory wire, said wire contracting whenheated, one portion of said shape memory wire fixed to a first body,another portion of said shape memory wire bearing on a second body, saidsecond body movable relative to said first body and movable relative tosaid shape memory wire, when heated said shape memory wire contracts andmoves said second body in a direction of travel from a first positionrelative to said first body toward a second position; a first magnetmounted to said first body and a second magnet mounted to said secondbody, said magnets generating a magnetic field, said magnetic fieldgenerating a magnetic force-acting-on and moving said second body insaid direction of travel to said second position after said shape memorywire initially contracts to decouple the force applied by said shapememory wire and move said second body relative to said shape memory wireto thereby unload the shape memory wire when the second body is moved tosaid second position; and said second magnet being offset from saidfirst magnet in a direction generally parallel to the direction oftravel of said second body, and said second magnet moving past saidfirst magnet when said second body moves in said direction of travel. 9.The actuator according to claim 8, wherein said first magnet extendsaround said second magnet.
 10. The actuator according to claim 8,wherein said wire is parallel to said direction of travel.
 11. Theactuator according to claim 8, wherein said memory wire comprises a pairof memory wires.
 12. The actuator according to claim 8, wherein saidshape memory wire comprises a first pair of shape memory wires, and saidactuator including a second pair of shape memory wires.
 13. The actuatoraccording to claim 8, further in combination with a valve, said valvehaving a valve stem, and said actuator selectively moving said valvestem.
 14. An actuator for selectively moving a part relative to anotherpart, said actuator comprising: a shape memory wire, said wirecontracting when heated, one portion of said shape memory wire fixed toa first body, another portion of said shape memory wire releasablycoupled to a second body, said second body movable relative to saidfirst body, when heated and coupled to said second body said shapememory wire contracts and moves said second body in a direction oftravel from a first positiOn relative to said first body toward a secondposition, and said direction of travel being parallel to said wire; anda magnetic field, said magnetic field generating a magnetic force, saidmagnetic force acting on said second body and moving said second body insaid direction of travel and decoupling the force applied by said shapememory wire and moving said second body relative to said shape memorywire wherein the length of contraction of said wire induces asubstantially equal longitudinal movement in said second body in saiddirection of travel, and said magnetic force moving said second beyondthe length of contraction of said wire.
 15. The actuator according toclaim 14, further comprising a first magnet mounted to said first bodyand a second magnet mounted to said second body, said magnets providingsaid magnetic field.
 16. The actuator according to claim 15, whereinsaid shape memory wire comprises a first shape memory wire and saiddirection of travel comprises a first direction of travel, said actuatorfurther comprising a second shape memory wire, said second shape memorywire contracting when heated, one portion of said second shape memorywire fixed at a second location of said first body and another portionof said second shape memory wire coupled to said second body whereinwhen heated said second shape memory wire contracts and moves saidsecond body toward said second location of said body in a seconddirection of travel opposed from said first direction of travel.
 17. Theactuator according to claim 14, further in combination with a valve,said valve having a valve stem, and said actuator selectively movingsaid valve stem.
 18. The actuator according to claim 17, wherein saidmemory wire extends through said valve stem.
 19. The actuator accordingto claim 17, wherein said valve comprises a three-way valve.